Which of these numbers is prime? ${13,\ 21,\ 45,\ 48,\ 63}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 13 are 1 and 13. The factors of 21 are 1, 3, 7, and 21. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 48 are 1, 2, 3, 4, 6, 8, 12, 16, 24, and 48. The factors of 63 are 1, 3, 7, 9, 21, and 63. Thus, 13 is a prime number.